I had this problem once, and it turned out the microsd card was not properly seated in it's slot. Just removing it and reinserting it making sure the retaining mechanism engaged fixed it for me.
You should still try to improve the wifi coverage as suggested. My printer currently resides at the edge of the covered wifi area, and it surely leads to performance issues (even with the Duet Web Control) when additional blocking factors (like people moving around or the printer frame being between the laptop and the access point).